Portable electronic devices such as MP3 players, cameras, and cell phones are typically fitted with a USB port, which allows them to transfer data to or from other controllers or hosts. Using these USB ports, portable devices may charge their batteries by conveniently drawing current from personal computers or other devices containing a USB port. In light of this convenience, engineers have developed standalone or dedicated USB chargers that provide a USB receptacle, allowing portable devices to charge either from a computer or a dedicated USB charger.
The advent of dedicated USB chargers has, however, increased charging complexity. Now, different types of USB charging ports are available, such as standard downstream ports, dedicated charging ports, and other such ports. Moreover, these ports allow portable devices to draw charge at different rates. Because these charging ports have different electrical characteristics, portable devices must identify the port type before they can begin drawing current. If the port is not correctly identified or not identified at all, a portable device may not be able to draw sufficient current from the port. Ascertaining the port type, however, can often become difficult.
To overcome this difficulty, the USB Battery Charging Working Group has introduced a Battery Charging Specification (BCS), which describes a charging port detection handshake profile, allowing BCS-compatible devices to detect the USB port type before drawing current. Though this profile allows compatible devices to charge, it introduces a problem for non-BCS compliant legacy devices.
Legacy products do not follow the BCS protocol; therefore, when these devices are connected to USB charger, they may not be able to properly detect the port type. Moreover, because legacy devices cannot identify the port type, they may be allowed to draw minimal current (80-100 mA) from the charging port or no current at all.
Moreover, smart charging ports (available today) support a large number of portable devices. These ports sometimes provide different handshaking protocols to downstream devices to initiate charging. Once a handshake is complete, the charging port provides current according to the successful handshake protocol's charging parameters. Some challenges exist, however, with this charging scheme. For example, if the portable device identifies the port as a standard downstream port, the handshake would be complete; but, the portable device may not be able to draw any current or may draw minimum current, which is not sufficient to charge the device efficiently. Moreover, when a legacy device connects to a BCS compatible port, the device cannot ascertain whether it is drawing the optimum amount of current.
Therefore, there remains a long-felt but unresolved need for a system or method for detecting charging current drawn by a downstream portable device during device charging or determining whether a portable device is charging.